High-efficiency Otto cycle engine with power generating expander

ABSTRACT

High-efficiency combustion engines, including Otto cycle engines, use a steam-diluted fuel charge at elevated pressure. Air is compressed, and water is evaporated into the compressed air via the partial pressure effect using waste heat from the engine. The resultant pressurized air-steam mixture then burned in the engine with fuel, preferably containing hydrogen to maintain flame front propagation. The high-pressure, steam-laden engine exhaust is used to drive an expander to provide additional mechanical power. The exhaust can also be used to reform fuel to provide hydrogen for the engine combustion. The engine advantageously uses the partial pressure effect to convert low-grade waste heat from engine into useful mechanical power. The engine is capable of high efficiencies (e.g. &gt;50%), with minimal emissions.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/338,637, filed Dec. 5, 2001, the entire teachings of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The burning of fuel to produce energy, and particularly mechanicalenergy, is at the root of modern society. Improvement in the efficiencyof such combustion, or in reduction of the emissions created bycombustion, are therefore important. A variety of prime movers or enginetypes are currently in use. The most widespread of these are theinternal combustion engine and the turbine.

The internal combustion engine, especially the spark-fired “Otto cycle”engine, is particularly ubiquitous, but presents significant challengesin the further improvement of its efficiency. The reciprocating pistonOtto cycle engine is in principle extremely efficient. For example, anOtto cycle engine operating with a 10:1 compression ratio, constantvolume TDC, no heat loss, and at constant specific heat ratio (K)should, in theory, have about a 60% cycle efficiency. However in actualpractice, engines typically operate at about half these air cycle values(i.e. about 31-32% efficiency). This is due to a number of reasons,including the fact that as the fuel burns, raising air temperature, thecombustion chemistry limits peak temperature through dissociation andspecific heat increase. Also, heat loss, finite burning, and exhausttime requirements reduce efficiency to about 85% theoretical fuel-aircycle values. Finally, engine friction, parasitic losses, etc., reduceactual power output by another 15% or so in a naturally aspiratedengine.

It is well-known that it would be more efficient to run such an engineleaner—i.e., at a higher stiochiometric ratio of oxygen to fuel—toimprove efficiency and reduce NOx (nitrogen oxide) emission. However,lean burning makes it difficult to sustain flame-speed (and thus avoidmisfire) in a conventional Otto cycle engine, which limits theeffectiveness of this approach. This problem could be overcome to someextent by “supercharging” the engine—i.e. running it at an inletpressure significantly above atmospheric pressure—but then the problemof premature detonation must be avoided, which limits the maximumavailable compression ratio, and thereby decreases the efficiency.

Moreover, each improvement in compression and leanness tends to increasethe creation of NOx at a given peak temperature, which must then beremoved by parasitic devices, such as exhaust emission systems. Further,the exhaust emission catalysts tend to be made inefficient, or poisonedentirely, by excess oxygen.

SUMMARY OF THE INVENTION

It has been discovered that the methods described herein can be used toincrease the efficiency of energy producing systems, particularlyengines, and more particularly the Otto cycle engine. The modificationsto present practice to achieve the improved process are relativelystraightforward and easily implemented, and produce significant andsynergistic effects when used in combination.

In one embodiment, a combustion engine power system comprises acombustion chamber which burns a fuel with a pressurized mixture ofsteam and air to produce useful power, waste heat, and asteam-containing exhaust stream; a compressor which pressurizes air toproduce a pressurized air stream; a water supply containing water thatis heated by waste heat from the power system and evaporated into thepressurized air stream to produce the pressurized mixture of air andsteam; a expander which is driven by the steam-containing exhaust streamto produce a power output in excess of the power required to pressurizethe air; and a power take-off of the excess power from the expander. Inone aspect, the present power generating system in effect superimposes aRankine or steam cycle power addition onto a conventionalturbo-compressor bottoming recuperation cycle. The steam cycle useswaste heat from the engine while simultaneously diluting the workingfluid (e.g. air) of the engine. This combination of the cycles (the“joint cycle”) improves cycle efficiency, suppresses detonation viasteam dilution, and increases engine specific power. In certainembodiments, the power system uses hydrogen to support flame propagationof the steam-diluted fuel-air mixture, and the hydrogen may beadvantageously provided by reforming a fuel using the high thermal masssteam-laden engine exhaust.

According to one aspect, the Otto cycle power system of the presentinvention operates with a steam-diluted fuel-air charge at an elevatedpressure. The working fluid of the engine (e.g. air) is compressed to ahigh-pressure by a compressor. The preferred pressure is in the range ofabout 2 to about 6 atmospheres, including pressures within this rangesuch as 2 to 3, 3 to 4, 4 to 5, and 5 to 6 atm. One embodiment describedherein uses a 4 atm pressurized air stream (1 atm=1 bar; 1 bar isapproximately 0.1 megapascal (MPa)).

Then, waste heat from the power system (such as from the engine exhaustor the engine cooling system) is used to evaporate water into thepressurized air to produce a pressurized mixture of air and steam. Thismay be efficiently done by partial pressure boiling of water (warmed bywaste heat of the engine) in the presence of the pressurized air streamat one or several locations in the system.

The pressurized steam-air mixture is then inducted into the combustionchamber of the engine, together with an appropriate amount of fuel,where they are combusted in the conventional fashion (i.e. two cycle orpreferably four cycle for maximum efficiency). The water (i.e. steam)concentration in the inlet stream of the combustion chamber should be ashigh as practical. In a 4 atm system, this can be about 8 moles of waterper mole of methane (or equivalent in gasoline).

One advantage in using a steam-diluted fuel-air mixture is a reductionin peak cycle temperature, which has the effect of improving cycleefficiency while also reducing NOx emissions. Another importantadvantage of operating dilute is the tremendous detonation suppressionresulting from the added steam. This makes it possible to operate theengine at high pressures (e.g. 4 atm). This turbocharging of the engineinlet not only aids in burning speed, but also provides the means forhybrid power/efficiency gains, and increases engine output andmechanical efficiency well over that of the natural aspiredstochiometrically correct standard engine practice.

Where the addition of steam diluent hampers the ability of the fuelmixture to burn in the engine, any conventional means for igniting adilute fuel-air mixture may be employed. In one embodiment, the primaryfuel injected into the combustion chamber is supplemented by theaddition of a second fuel, such as hydrogen, to help sustain flame-frontpropagation in the steam-diluted mixture. Moreover, by turbocharging theengine, the resultant high-temperature and high-pressure exhaust can beadvantageously used as a source of heat and/or steam to partially reformthe primary fuel to provide a source of the supplemental fuel (e.g.hydrogen). Because the exhaust contains a substantial amount of steam,the exhaust itself can provide steam required for the reformingreaction. Alternatively, or in addition, steam from elsewhere in thesystem, such as a dedicated boiler, can be used.

The combustion in one or more combustion chambers (or cylinders)provides the primary output power of the system, and is typically useddirectly for mechanical work, or indirectly for electricity generation.The engine combustion also generates waste heat, some of which iscontained in the high-temperature engine exhaust, and some of which isremoved from the engine via a cooling fluid which circulates through theengine. Much of this waste heat, such as heat from the engine coolingloop and heat from low-temperature exhaust, is low-grade heat that isnotoriously difficult to recapture in a useful manner. Consequently, ina conventional engine, this low-temperature waste heat is typicallyrejected from the engine.

In the present invention, however, at least a portion of thislow-temperature waste heat is advantageously recaptured by using theenergy of the waste heat to evaporate water into the pressurized engineoxidant (e.g. air) to produce a pressurized steam-air stream having asignificant expansion potential. This expansion potential can be used toproduce additional mechanical energy, and thereby improve engineefficiency, as described below. In general, as much warm water should beevaporated to recover its latent heat as can be accommodated by thepressurized air. The proportion of the latent heat that is recovered assteam depends on the type of system and on its details. A proportion ofat least about 50% is desirable, and generally obtainable. With atypical Otto cycle engine, recovery in the range of about 50% to 75% isoften obtainable. Recoveries significantly below 50%, for example belowabout 25%, while still beneficial in terms of efficiency, may not besufficient to justify the extra cost in constructing the system of theinvention.

After combustion, the exhaust stream from the combustion chamber is at ahigh-temperature (e.g. 2100° Rankin, or about 1200° K.) and is still atthe elevated system pressure (e.g. 4 atm). The exhaust is loaded withsteam, and has a substantial expansion potential that can beadvantageously utilized to drive an expander, (preferably a turbine butnot limited thereto) to produce a power output. A power take-off fromthe expander can be utilized, for example, to drive an electricalgenerator, or to gear the expander power output into the primary poweroutput from the engine. The expander can also be coupled to and used todirectly drive the air-input compressor.

In contrast to conventional turbo-compressor (Brayton) cycle engines,the present invention is able to generate significant excess power bythe expansion of steam-laden engine exhaust. The steam provides anadditional mass flow through the expander, for example, twice the“specific mass flow” (i.e. specific-heat adjusted mass flow) of the airalone. In effect, the present invention adds a Rankine, or steam cycle,power addition to the conventional turbo-compressor bottomingrecuperation cycle. Thus, instead of simply recouping the power expendedin compressing the air, the “joint” Brayton/Rankine cycle of the presentinvention is able to generate significant additional power. In a 4 atm.system, for example, the expander can produce over three times, and insome cases over four times, the power that is required to drive thecompressor. This excess power can be significant in terms of overallsystem efficiency, and can amount to a 33% increase in net power outputof the system as a whole.

Moreover, this excess power of the turbine can be obtained at little orno cost, as it is derived from the recovery of low temperature “waste”heat via evaporation of warm water into pressurized air (i.e. the“partial pressure effect”). The energy gained is essentially the latentheat consumed to vaporize water. The latent heat is a significantquantity: it takes about 2326 joules per gram to evaporate water at 60°C., while it takes only about an additional 1465 joules per gram to heatthe evaporated water (steam) by an additional 800° C. The sequence ofpressurization of air before evaporation of water is important in orderto maximize efficiency improvements, because while significant energy isexpended to compress the air, very little energy is required to compressthe warm water to the same pressure.

A typical range of concentration of steam in the system exhaust is inthe range of about 30 to 60% by weight, preferably in the range of about30% to 50% by weight, and even more preferably in the range of about 33%to 45% by weight (for example, about 520 lbs of steam in 720 lbs of air,or about 40%). A lower end of the range is typically about 20 to 25%,which is both the general range in which the presence of steam in thepressurized fuel-air mixture begins to require the presence of hydrogen(or similar means) for reliable ignition, and about the lower limit atwhich the extra complexity of the “joint cycle” engine is repaid byimprovements in efficiency. Steam concentrations above 50% are desirablewhen they can be readily obtained. At very high levels of steam, such asabout 75% by weight and above, the combustion of the fuel-steam-airmixture can become more difficult, and the loss in power becomes alimiting factor on maximum percent of steam incorporated, with theprecise limit depending on the details of the system design.

After expansion, the expanded and cooled exhaust can next be used toprovide heat to evaporate or preheat water. When sufficiently cool, itis passed through a condensing radiator to condense water. The recoveredwater is then recycled to provide water for making steam. The condensingradiator is optionally combined with the radiator used for cooling theengine, after the engine cooling fluid has likewise been used toevaporate or preheat water. Stoichiometric operation of the enginemaximizes the condensing exhaust dewpoint. In order to maintainsufficient water recovery levels in varying ambient temperatures andclimatic conditions, the exhaust dewpoint can be adjusted by selectivelyapplying a backpressure to the exhaust (e.g. via a flow-restrictingvariable valve) as needed.

According to one aspect, heat from the high-temperature engine exhaustcan be used to partially reform gasoline or other fuel, preferably inthe presence of steam, to produce a mixture of hydrogen and combustiblecarbon-containing materials. Heat required for the reforming reaction ispreferably provided by heat exchange with the highest temperatureexhaust gases (i.e. immediately or soon after the exhaust leaves thecombustion chamber). Steam, if required for the reforming process, maybe obtained by injecting steam from a steam source, or by using aportion of the steam-laden exhaust itself as a steam source. Optionally,oxygen can be injected at this stage as well. When the steam-ladenexhaust itself is used for the reforming, the portion of the exhaustrequired will vary according to the exact design and could be in therange of about 35% to about 5%, depending on the steam content of theexhaust. About 10% is optimal for the 4 atm. supercharge.

In one embodiment, the steam for the fuel reforming reaction can be madeby boiling water using heat from the exhaust at a cooler portion of theexhaust stream (e.g. below the expander). Even after the exhaust streamis expanded and cooled by the expander, there is still enough heatremaining to boil some steam undiluted at atmospheric pressure. Thissteam, along with the fuel to be reformed, can then be supplied to areforming zone that is heated by the high-temperature exhaust (i.e.before expansion) to support the endothermic fuel reforming reaction.

The hydrogen-containing reformate generated from the exhaust can beadvantageously supplied to the combustion chamber by passing all of thefuel through the reformer, without necessarily reforming all of the fuelcompletely. Alternatively, a reformate can be used as a supplement tothe primary fuel source, which generally comprises partially reformedand/or unreformed fuel. The presence of the hydrogen in the fuel mixtureallows sufficient flame speed to support the lean, dilute combustiondescribed above. It may be less important to supply a hydrogen fuelcharge for other types of combustion. Reforming the fuel by steamreforming (reaction of fuel with water to produce hydrogen and otherproducts)—including variant forms of autothermal reforming (ATR) andpartial oxidation (POx)—is preferred. Formation of hydrogen by simpleheating of fuel (“cracking”) is known, and is also useable in theinvention wherever the tendency to produce carbon deposits can becontrolled. In principle, a store of pure hydrogen or of hydrogen mixedwith another gas could also be used, although it would be less practicalin most applications.

The systems and methods described herein can advantageously be used toprovide a combustion engine characterized by high-efficiency and lowemissions. For example, employing the principles of the presentinvention, a standard off-the-shelf Otto-cycle engine can perform atincreased specific power with a nominal 52% efficiency, while at thesame time having only trace emissions.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a schematic of an Otto cycle engine system according to oneembodiment of the invention; and

FIG. 2 is a schematic of a second embodiment of an Otto cycle enginesystem having a steam boiler.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the schematic illustration of FIG. 1, it will be moreclearly understood how the combination of steam generation, hydrogengeneration, stoichiometric air combustion, and elevated dew point waterrecycle synergistically work together in an engine of the invention. Theturbocharger compounded engine of this example uses exhaust reformingand steam generation via the partial pressure effect from the waste heattemperature sinks of the engine. The latent energy of this waste heat istransferred to the pressurized air of the engine, where it can be usedfor power generation. The following example contains specific amounts ofinputs and values of variables (temperature, pressure, etc) in order toprovide an example of the efficiency improvement possible with thepresent invention. These specific examples are not to be taken aslimiting the scope of the invention.

As shown in FIG. 1, the Otto cycle engine includes a compressor 200,which is preferably a two-stage compressor. At State 1 (i.e. S₁), an airflow 204 inducted from the atmosphere and consisting of 774 Lbs/hr (1lb=0.456 kg; 1 hr=3600 sec.) is compressed by the compressor 200 to 4atm. The air temperature rises to about 410° F. (ca. 210° C.), assuminga 75% efficiency of the compressor and power consumption of 18 kW.

Starting with the induction air 204, water for vaporization in the airis added in three separate steps in this example (in other embodiments,water can be added in more or fewer steps). First, an initial waterinput 202 is added sometime before, during, or preferably aftercompression to yield, at the compressor outlet 206, a pressurized fluidstream at State 2 (S₂,), wherein T=250° F., leading to 25 lbs. of waterbeing evaporated into the stream. At S₂, the degree of saturation of theair by water, w_(s)=0.0323, the dew point, T_(s)=141° F., and thepartial pressure of steam, P_(s)=2.9 psia. (1 psi=ca. 7 kPa).

After exiting the compressor 200, the moist air at S₂ enters a firstpartial pressure boiler 210 for counter flow heat exchange with theturbine exhaust 238. Before or at the entrance to the partial pressureboiler 210, a second water addition is made at 208. Heat transferredfrom the engine exhaust evaporates or boils about 240 lbs. of additionalwater into the 774 lbs. of air (plus 25 lbs of water) at 4 atm pressure,raising the air dew point from about T_(s)=139° to T_(s)=230° F., andthe saturation, w_(s), to 0.34, resulting in a total of 265 lbs. ofsteam present in the original 774 lbs. of compressed air at 212, (stateS₃). The heavily moisture laden exhaust, from which heat transfer hasbeen made, drops from about 952° F. at the inlet of the partial pressureboiler to a temperature of about 250° to 300° F., (ca. 120 to 150° C.),typically with a small amount of water condensation (State 9; locationat 240).

In this example, a third steam addition is made between states S₃ (at212) and S₄ (at 219), accompanied by heat transfer from an enginecooling loop. In the particular embodiment illustrated, this is donethrough direct contact transfer, under partial pressure conditions, ofheat from the water cooling loop of the engine. Engine cooling water218, which may be the primary coolant or may be a secondary loop heatedby a primary coolant loop (such as, for example, a primary loopcontaining antifreeze), is injected into a second partial pressureboiler 214, for example by spraying, and equilibrated with the air/steammixture 212 entering from the first boiler 210 (S₃). Spraying may bereplaced or supplemented by other methods of mixing vapor and liquid,including passage over columns of porous materials (as in distillation),by thin film evaporation, etc. Any of the known methods and apparatusthat are operable at these temperatures and pressures, and preferablyones which are physically compact, can be used.

The exiting stream at 219 (S₄) has acquired about 156 lbs. of additionalsteam, generated by evaporation as the engine coolant is cooled from280° F. to about 260° F. The air/steam enters the engine inlet 220 atabout T_(s)=244° F. carrying about 421 lbs. of steam. Non-evaporatedcoolant is returned to the engine via conduit 216. Engine cooling water(primary or secondary) is kept at a constant volume by the addition ofwater into the cooling loop; illustration of this step is omitted forclarity.

Note that this particular heat-mass transfer process, in addition toexhaust heat transfer, is one characteristic of this system to providehigh efficiency. Here, a heat source temperature capable of producing,in a closed Rankine steam cycle, only about a one atm pressure dropthrough a turbine, has been used to raise the power availability to 3atm pressure drop by the mechanism of boiling water in air—a “partialpressure” benefit. A burden is created in that the evaporated water willeventually need to be recovered from the engine exhaust using acondensing radiator. This burden is partially offset later, however, bygains in cycle power and efficiency.

At state S₄, the air/steam mixture 219 comprises the original 774 lb aircharge at 4 atm., and now further contains over 421 lbs. steam, withsaturation w_(s)=0.546, and T_(s)=244° F. The air/steam mixture hascaptured a substantial portion of the engine's waste heat. Thissteam/air mixture 219 is now combined with the fuel, preferably at anessentially stoichiometric ratio. The fuel has also been partiallyreformed, as described below.

At state S₅, the engine receives an inlet charge of chemically correctfuel-air, with 54.6% mass dilution with steam, or with specific heatcorrections, about 100% of thermal dilution—the pressure equivalent ofoperating an engine at 200% of stoichiometric air charge. Multiplyingthe fuel heating value by 1.12 (due to the effects of the endothermicreforming reaction described below) yields an equivalent F/F_(c)=0.56(where F/F_(c) equals the fuel-to-air ratio, F, divided by thechemically correct fuel-to-air ratio F_(c). F_(c) is 1 for a normallyaspirated engine, but is 0.5 here because of the steam dilution.)Operation under these conditions is difficult without having hydrogen aspart of the fuel charge to provide good flame front propagation.Additional benefits of the high steam content include a fuel-air cycleefficiency of approximately 47%, a steam corrected compression ratioequivalent R=8, and at most only trace levels of NOx emissions.

Peak cycle temperature T₃ in the combustion chamber is around 4300° R.At the end of the power stroke before exhausting and blowdown, thecombustion temperature is calculated as being about 2400° R. (ca 1940°F.; ca. 1060° C.). Because the elevated exhaust pressure of 4 atm limitsblowdown, the actual exhaust temperature is closer than usual to thecalculated value. Exhaust manifold temperature is around 2100° R. (ca.900° C.). The engine consumes essentially the entire stoichiometricoxygen charge, generating an additional 101 lbs of steam. The engineexhaust 222 at state S₆ is P=4 atm, T=2100° R. (ca. 1640° F.; ca. 900°C.), with the gas now containing 717 lbs of CO₂ and N₂ (and nosignificant oxygen content), 522 lbs of steam, and a saturation, W_(s),of 0.728.

Per mole of methane or equivalent supplied, the exhaust has a molarcomposition of about 1 CO₂, 7.52 N₂, and 10 H₂O. This is five times thesteam generated by normal stoichiometric combustion with no diluent. Theexhaust is loaded with thermal mass and steam, and is suitable for usefor turbine power and optionally for steam reforming.

In a preferred mode, between S₆ (222) and S₇ (230) about 10% of thisexhaust is diverted at 228 and mixed with the incoming fuel from point226 (which is treated as if it were CH₄ for simplicity of calculation).This mixture is introduced into an “exhaust reformer” 224 that is heatedby thermal transfer from the remaining exhaust stream. The reactionbetween the exhaust and the fuel in the exhaust reformer is preferablyaccelerated by a reforming catalyst.

In an alternative embodiment, illustrated in FIG. 2, which is otherwiseidentically numbered, the steam required for partial fuel reforming issupplied by a full pressure boiler 260 supplied by water from a source262. Heat from the expanded exhaust 238 creates steam, which is conveyedthrough conduit 264 to mix with the fuel 226 at or near the entrance tothe exhaust reformer 224.

Given a desired 50% methane slip in the reforming reaction, the overallreaction is, on a molar basis:

 1CH₄+0.1CO₂+0.752N₂+1H₂O→0.5CH₄+0.1CO₂+1.5H₂+0.5H₂O+0.752N₂

Q_(ch4)=344,000 BTU/Lbs Q_(ref)=385,000 BTU.

After the fuel reforming, the exhaust temperature drops by about 261°F., yielding 1380° F. at S₇, point 230, but the heating value of thefuel has been increased by about 12% by the endothermic conversion ofmethane and water (and absorbed heat) to hydrogen and carbon monoxide.

Returning to the engine, an efficiency number can now be calculated forthis example. Before this, however, one more parameter should beconsidered. The engine, when normally aspirated (i.e., not pressurized),classically runs at 85% mechanical efficiency. The present engineoperates with a dilute charge, which reduces power per unit air by about50%. In compensation, the induction pressure may be increased to 4 atm,which increases power by about 3.7 times when corrected for manifoldtemperature. In addition, 12% heating value is added by reforming. Sothe nominal indicated power is approximately doubled with essentiallythe same engine friction and parasitics. Hence, an engine that wouldnormally be rated at 50 kW can produce 114 kW, without prematuredetonation in the cylinders due to the suppressive effect of the steam.

From standard fuel-air cycle curves, with heating value correction, 85%cycle performance efficiency, and 90% mechanical efficiency, there is a47%×1.12×0.85×0.9=40.2% efficiency, at this point in the cycle, comparedto a 36% efficiency without the features of the partial pressure boilingcycle. The increase in efficiency is believed to be in large part due toa combination of the successful dilute combustion at pressure, therecycling of exhaust heat via reforming, and the capture of waste heatas steam. (Note that in this example, the engine efficiency hasincreased to 40% even before expansion of the exhaust).

The exhaust at 230, state S₇, optionally and preferably travels througha cleanup catalyst 232 at about 1300° F. and 4 atm for hydrocarbonemission prevention, which is still likely to be required. Note that ifNOx reduction is desired, the well-known three-way catalyst commonlyused in automobile applications can be used here to further reduce NOx,because the exhaust has the required chemically correct (i.e. nearlyoxygen-free) constitution. This is in contrast to diesels, gas turbines,and some fuel-cell burners, which cannot use inexpensive catalystsbecause there is significant oxygen in the exhaust stream.

Between State S₇ (at 230) and state S₈ (at 238), an expander 234, here aturbine, expands the exhaust gas and steam charge from about 4 atm toabout 1 atm at about 85% efficiency. The temperature drop is about 454°F., leaving about 952° F. as the temperature of the remaining exhaust at238. In this example, the turbine produces an output power of 59 kW(where the turbine power is equal to the temperature drop multiplied bythe sum of the (mass flow×specific heat) for each of the exhaustgases—i.e.454_(ΔT)×[(123×0.4)_(CO2)+(594×0.24)_(N2)+(522×0.5)_(H20)]/0.3412_((Conversion Factor))=59kW.) This 59 kW power output more than compensates for the powerrequired for air compression, which is about 18 kW. The turbine mayoptionally be used to drive the air compressor 200, and produces excesspower through generator 236. Generator 236 can optionally be amotor/generator, using electric power from a battery to start up thesystem; or, a compressor/motor and a turbine/generator can be separateunits (not illustrated), with a slight loss of efficiency.

In addition, or as an alternative to the use of a generator, the outputpower of the turbine can be directly added to the engine power output,such as by direct addition of the torque of the turbine to that of theengine shaft, by a spur wheel attachment, for instance.

In state S₈, at point 238, the exhaust is at about T=952° F. (about 500°C.), water saturation of the gas stream w_(s) is 522/717=0.728, and thusdew point=182° F., and pressure=1 atm. This gas enters the first partialpressure boiler 210 for heat transfer to the charge of induction airmixed with water. Recall previously that the partial pressure effectmeans that boiling or evaporation begins with the induction air inlet atT_(s)=about 140° F. and ends with T_(s)=228° F. With adequate heatexchange area, the steam generation quantity stated before, about 240lbs in the induction air, is conservative considering the sensibletemperature drop of the exhaust gas. In fact, exhaust gas condensingwould occur with an exhaust exit temperature of even 165° F. Thus, ifhalf the exhaust water condensed, it would add in theory twice theboiling heat flux into the induction air/water mixture that was assumedabove. Hence, the above calculations are definitely conservative interms of the amount of heat that can be recovered as steam.

Finally, at state S₉ the exhaust enters the condensing radiator 244,which has a fan 246, for working fluid (water) recycle. Since the systemis operated at a chemically correct stoichiometry, the outputtemperature at the final exhaust state S₁₀ at 248 can be as high as 132°F. and still produce water balance, i.e., deposit enough water in thewater recycle collector 242 to provide the water that is added to thecompressed air at 202, 208 and 214. (The water recycling system, whichwill include at least one pump, and may include a water purificationapparatus, is not illustrated.) If feasible, a lower exhaust exittemperature is preferred. Since the exhaust enters the radiator atT_(s)=165° F. or above, heat transfer is “wet”, i.e., the radiator tubescontacting the exhaust have a coating of water, and so is high rate andnon-corrosive, which favors durability of the radiator.

The final result of the partial pressure hybrid Otto cycle engine ofthis example is as follows:

Power=114 kW engine plus 59 kW turbine less 18 kW compressor=155 kW

Efficiency=40% (engine)×155/114=54%

Radiator Load=133.60 kW Engine Size=50 kW (standard).

It will be understood that various modifications can be made to thesystem described above without departing from the scope of theinvention. For example, in the embodiment described above, the enginecoolant water is evaporated into the pressurized air-steam stream in aseparate partial pressure boiler. However, in other embodiments, theengine coolant can be boiled in the engine block itself, at saturation,so that a two-phase steam/water mixture is introduced into the alreadyhumidified air. In this way, even more evaporation can be obtained,putting more steam into the cylinders.

Also, it is important to ensure that the dew point in the exhaust ishigh enough to permit efficient water recovery. When ambienttemperatures are low, for example 25° C. or less, then condensation ofwater from a 60° C. exhaust stream is easy to achieve. However, whenambient temperatures reach higher temperatures, such as 40° C., waterrecovery becomes more difficult. The usual solution to this problem isto size the radiator for the worst expected case of ambient temperature,but this can be awkward and expensive, especially in a mobile system.Because the system of the present invention is pressurized, analternative approach can be used. At high ambient temperatures, abackpressure can be selectively imposed on exhaust outlet 248 by, forexample, a flow-restricting variable valve 270. The backpressure raisesthe dew point of the exhaust stream (because the saturation volumetricconcentration of water in air decreases with increasing air pressure),thus making the water in the exhaust more easily recoverable. Forexample, if a system is operated at 4 atmospheres, a backpressure of 0.5atmospheres can increase the dew point by 10 to 20° C., which allowsefficient recovery at higher ambient temperatures without increase ofradiator size. There is a penalty for the backpressure in terms ofdecreased system efficiency, since there is less pressure drop throughthe expander. However, back pressure can be regulated to be the minimumrequired to recover sufficient water under ambient conditions, thusallowing the system—for example, in an automobile—to operate undervarious temperature and climatic conditions while maintaining themaximum efficiency possible under those conditions.

The “joint cycle” engine of the invention can be operated with orwithout a conventional closed-loop radiator for the engine coolingsystem, in addition to the condensing radiator for the engine exhaustdescribed above. A conventional radiator may not be necessary, forinstance, where a sufficient amount of the engine waste heat can berecovered by evaporation of water into pressurized air.

The above worked example uses an Otto cycle engine as a basis forimprovement. Heat energy recovery is also applicable to other types ofprime movers, although the efficiency gains may be smaller. For example,a similar arrangement can in principle be used in a diesel engine. Theincrease in efficiency would likely be smaller, because the diesel isalready more efficient in terms of combustion temperatures, is typicallyalready pressurized to some extent, and will be adversely affected inits compression by a charge containing a high level of steam. However,an efficiency benefit of recovering heat energy from the exhaust andoptionally from the engine coolant by using the heat to make steam inpressurized air, and converting this heat energy to mechanical energyvia an expander, is still applicable.

The invention may also be particularly advantageous when used forapplications having a constant operating speed, such as a hybrid(gas/battery) car engine, and certain types of domestic co-generationsystems. In these cases, the turbine can be optimized for the operatingspeed of the engine.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A combustion engine power system comprising: acombustion chamber which burns a fuel with a pressurized mixture ofsteam and air to generate useful power and waste heat, the combustionchamber producing a steam-containing exhaust stream; a compressor influid communication with the combustion chamber, the compressorpressurizing air to produce a pressurized air stream; a water supply influid communication with the pressurized air stream, at least a portionof the water from the supply being heated by waste heat from the powersystem and evaporating into the pressurized air stream to produce thepressurized mixture of air and steam; an expander in fluid communicationwith the combustion chamber, the expander being driven by thesteam-containing exhaust stream to produce a power output in excess ofthe power required to pressurize the air; and a power take-off of theexcess power from the expander, the combustion engine power systemcomprising at least one of an Otto cycle engine and a Diesel engine. 2.The system of claim 1, wherein the power take-off is used to drive anelectrical generator.
 3. The system of claim 1, wherein the expandercomprises a turbine.
 4. The system of claim 1, comprising multiple watersupplies that are evaporated into the pressurized air, wherein at leastone water supply is heated by waste heat from the exhaust stream.
 5. Thesystem of claim 1, wherein the engine comprises an Otto cycle engine. 6.The system of claim 1, wherein the engine comprises a Diesel engine. 7.The system of claim 1, wherein at least about 25% of the energy of thewaste heat generated by the power system is expended in the evaporationof the water into the pressurized air.
 8. The system of claim 1, whereinat least about 50% of the energy of the waste heat generated by thepower system is expended in the evaporation of the water into thepressurized air.
 9. The system of claim 1, wherein the steam-containingexhaust used to drive the expander comprises at least about 20% steam byweight.
 10. The system of claim 1, wherein the steam-containing exhaustused to drive the expander comprises at least about 33% steam by weight.11. The system of claim 1, wherein the output power from the expanderdrives the compressor.
 12. The system of claim 1, wherein the waste heatwhich heats the water comprises waste heat from the exhaust stream. 13.The system of claim 12, wherein the waste heat from the exhaust streamheats the water after the exhaust stream drives the expander.
 14. Thesystem of claim 1, wherein the compressor pressurizes the air to apressure between about 2 and 6 atmospheres.
 15. The system of claim 14,wherein the pressure is approximately 4 atmospheres.
 16. A combustionengine power system comprising: a combustion chamber which burns a fuelwith a pressurized mixture of steam and air to generate useful power andwaste heat, the combustion chamber producing a steam-containing exhauststream; a compressor in fluid communication with the combustion chamber,the compressor pressurizing air to produce a pressurized air stream; awater supply in fluid communication with the pressurized air stream, atleast a portion of the water from the supply being heated by waste heatfrom the power system and evaporating into the pressurized air stream toproduce the pressurized mixture of air and steam; an expander in fluidcommunication with the combustion chamber, the expander being driven bythe steam-containing exhaust stream to produce a power output in excessof the power required to pressurize the air; and a power take-off of theexcess power from the expander, the power take-off being geared into thesystem output of the engine.
 17. A combustion engine power systemcomprising: a combustion chamber which burns a fuel with a pressurizedmixture of steam and air to generate useful power and waste heat, thecombustion chamber producing a steam-containing exhaust stream; acompressor in fluid communication with the combustion chamber, thecompressor pressurizing air to produce a pressurized air stream; a watersupply in fluid communication with the pressurized air stream, at leasta portion of the water from the supply being heated by waste heat fromthe exhaust stream of the power system and evaporating into thepressurized air stream to produce the pressurized mixture of air andsteam; a partial pressure boiler which evaporates water into thepressurized air stream, and is heated by heat exchange with the exhauststream; an expander in fluid communication with the combustion chamber,the expander being driven by the steam-containing exhaust stream toproduce a power output in excess of the power required to pressurize theair; and a power take-off of the excess power from the expander.
 18. Acombustion engine power system comprising: a combustion chamber whichburns a fuel with a pressurized mixture of steam and air to generateuseful power and waste heat, the combustion chamber producing asteam-containing exhaust stream; a compressor in fluid communicationwith the combustion chamber, the compressor pressurizing air to producea pressurized air stream; a water supply in fluid communication with thepressurized air stream, at least a portion of the water from the supplybeing heated by waste heat from the power system and evaporating intothe pressurized air stream to produce the pressurized mixture of air andsteam, the water supply being heated by an engine cooling system; anexpander in fluid communication with the combustion chamber, theexpander being driven by the steam-containing exhaust stream to producea power output in excess of the power required to pressurize the air;and a power take-off of the excess power from the expander.
 19. Thesystem of claim 18, further comprising a partial pressure boiler whichevaporates water into the pressurized air stream, the water being heatedby the engine cooling system.
 20. A combustion engine power systemcomprising: a combustion chamber which burns a fuel with a pressurizedmixture of steam and air to generate useful power and waste heat, thecombustion chamber producing a steam-containing exhaust stream; acompressor in fluid communication with the combustion chamber, thecompressor pressurizing air to produce a pressurized air stream;multiple water supplies in fluid communication with the pressurized airstream, at least a portion of the water from the supplies being heatedby waste heat from the power system and evaporating into the pressurizedair stream to produce the pressurized mixture of air and steam, at leastone water supply being heated by an engine cooling system; an expanderin fluid communication with the combustion chamber, the expander beingdriven by the steam-containing exhaust stream to produce a power outputin excess of the power required to pressurize the air; and a powertake-off of the excess power from the expander.
 21. The system of claim20, wherein at least one water supply is heated by waste heat from theexhaust stream.
 22. A combustion engine power system comprising: acombustion chamber which burns a fuel with a pressurized mixture ofsteam and air to generate useful power and waste heat, the combustionchamber producing a steam-containing exhaust stream, the fuel comprisinga fuel mixture comprising a first fuel component and a second fuelcomponent, the first fuel component facilitating burning of the secondfuel component in the presence of steam; a compressor in fluidcommunication with the combustion chamber, the compressor pressurizingair to produce a pressurized air stream; a water supply in fluidcommunication with the pressurized air stream, at least a portion of thewater from the supply being heated by waste heat from the power systemand evaporating into the pressurized air stream to produce thepressurized mixture of air and steam; an expander in fluid communicationwith the combustion chamber, the expander being driven by thesteam-containing exhaust stream to produce a power output in excess ofthe power required to pressurize the air; and a power take-off of theexcess power from the expander.
 23. The system of claim 22, wherein thefirst fuel component comprises hydrogen.
 24. The system of claim 23,wherein the hydrogen is produced by reforming a fuel using at least oneof heat and steam from the engine exhaust.
 25. The system of claim 22,wherein the first fuel component comprises a product of a reformingreaction, and the second fuel component comprises at least one ofpartially reformed fuel and un-reformed fuel.
 26. The system of claim25, further comprising a fuel reformer for reforming fuel, the fuelreformer heated by heat exchange with the engine exhaust.
 27. The systemof claim 26, wherein steam for a fuel reforming reaction is provided atleast in part by diverting a portion of the steam-containing engineexhaust to the fuel reformer.
 28. The system of claim 26, wherein steamfor a fuel reforming reaction is provided at least in part by a boilerhaving a water supply, where water in the boiler is heated by engineexhaust to produce steam.
 29. A combustion engine power systemcomprising: a combustion chamber which burns a fuel with a pressurizedmixture of steam and air to generate useful power and waste heat, thecombustion chamber producing a steam-containing exhaust stream; acompressor in fluid communication with the combustion chamber, thecompressor pressurizing air to produce a pressurized air stream; a watersupply in fluid communication with the pressurized air stream, at leasta portion of the water from the supply being heated by waste heat fromthe power system and evaporating into the pressurized air stream toproduce the pressurized mixture of air and steam; an expander in fluidcommunication with the combustion chamber, the expander being driven bythe steam-containing exhaust stream to produce a power output in excessof the power required to pressurize the air; a power take-off of theexcess power from the expander; and a condensing apparatus forrecovering water from the engine exhaust prior to discharging theexhaust from the system.
 30. The system of claim 29, further comprisingan apparatus for selectively applying a backpressure to the engineexhaust to facilitate recovery of water.
 31. A combustion engine powersystem comprising: a combustion chamber which burns a fuel with apressurized mixture of steam and air to generate useful power and wasteheat, the combustion chamber producing a steam-containing exhauststream; a compressor in fluid communication with the combustion chamber,the compressor pressurizing air to produce a pressurized air stream; awater supply in fluid communication with the pressurized air stream, atleast a portion of the water from the source being heated by waste heatfrom the power system and evaporating into the pressurized air stream toproduce the pressurized mixture of air and steam; an expander in fluidcommunication with the combustion chamber, the expander being driven bythe steam-containing exhaust stream to produce a power output; and acondensing apparatus for recovering water from the engine exhaust priorto discharging the exhaust from the system.
 32. The system of claim 31,further comprising an apparatus for selectively applying a backpressureto the engine exhaust to facilitate recovery of water.
 33. A combustionengine power system comprising: a combustion chamber which bums a fuelmixture with a pressurized mixture of steam and air to generate usefulpower and waste heat, the fuel mixture comprising at least two fuelcomponents, wherein a first fuel component facilitates combustion of asecond fuel component in the presence of steam, the combustion chamberproducing a steam-containing exhaust stream; a compressor in fluidcommunication with the combustion chamber, the compressor pressurizingair to produce a pressurized air stream; a water supply in fluidcommunication with the pressurized air stream, at least a portion of thewater from the supply being heated by waste heat from the power systemand evaporating into the pressurized air stream to produce thepressurized mixture of air and steam; and an expander in fluidcommunication with the combustion chamber, the expander being driven bythe steam-containing exhaust stream to produce a power output.
 34. Thesystem of claim 33, wherein the first fuel component comprises hydrogen.35. The system of claim 34, wherein the hydrogen is produced byreforming a fuel using at least one of heat and steam from the engineexhaust.
 36. The system of claim 33, wherein the first fuel componentcomprises a product of a reforming reaction, and the second fuelcomponent comprises at least one of partially reformed fuel andun-reformed fuel.
 37. The system of claim 36, further comprising a fuelreformer for reforming fuel, the fuel reformer heated by heat exchangewith the engine exhaust.
 38. The system of claim 37, wherein steam for afuel reforming reaction is provided at least in part by diverting aportion of the steam-containing engine exhaust to the fuel reformer. 39.The system of claim 37 , wherein steam for a fuel reforming reaction isprovided at least in part by a boiler having a water supply, where waterin the boiler is heated by engine exhaust to produce steam.
 40. A methodof operating a combustion engine power system comprising: compressingair to provide a pressurized air stream; evaporating water into thepressurized air stream, using waste heat from the power system, toproduce a pressurized steam-air mixture; burning fuel with thepressurized steam-air mixture in a combustion chamber to produce usefulpower and waste heat, the burning also producing a steam-containingexhaust stream; expanding the steam-containing exhaust stream through anexpander to produce an output power in excess of the power required toprovide the pressurized air stream; and taking-off excess power from theexpander, the combustion engine power system comprising at least one ofan Otto cycle engine and a Diesel engine.
 41. The method of claim 40,wherein taking-off excess power comprises using the power to drive anelectrical generator.
 42. The method of claim 40, wherein at least about25% of the energy of the waste heat generated by the power system isexpended in the evaporation of water into the pressurized air.
 43. Themethod of claim 40, wherein at least about 50% of the energy of thewaste heat generated by the power system is expended in the evaporationof water into the pressurized air.
 44. The method of claim 40, whereinthe steam-containing exhaust used to expand the expander comprises atleast about 20% steam by weight.
 45. The method of claim 40, wherein thesteam-containing exhaust used to expand the expander comprises at leastabout 33% steam by weight.
 46. The method of claim 40, wherein theexpander power is used to compress the air.
 47. The method of claim 40,wherein the air is compressed to a pressure between about 2 and 6atmospheres.
 48. The method of claim 47, wherein the air pressure isabout 4 atmospheres.
 49. The method of claim 40, wherein the waste heatcomprises waste heat from the exhaust stream.
 50. The method of claim49, wherein the waste heat is transferred from the exhaust stream afterthe exhaust has expanded in the expander.
 51. A method of operating acombustion engine power system comprising: compressing air to provide apressurized air stream; evaporating water into the pressurized airstream, using waste heat from the power system, to produce a pressurizedsteam-air mixture; burning fuel with the pressurized steam-air mixturein a combustion chamber to produce useful power and waste heat, theburning also producing a steam-containing exhaust stream; expanding thesteam-containing exhaust stream through an expander to produce an outputpower in excess of the power required to provide the pressurized airstream; and taking-off excess power from the expander and gearing theexpander power output into the system power output from the engine. 52.A method of operating a combustion engine power system comprising:compressing air to provide a pressurized air stream; evaporating waterinto the pressurized air stream, using waste heat from the power system,to produce a pressurized steam-air mixture, at least a portion of thewater evaporated into the pressurized air stream being warmed by wasteheat from an engine cooling system; burning fuel with the pressurizedsteam-air mixture in a combustion chamber to produce useful power andwaste heat, the burning also producing a steam-containing exhauststream; expanding the steam-containing exhaust stream through anexpander to produce an output power in excess of the power required toprovide the pressurized air stream; and taking-off excess power from theexpander.
 53. A method of operating a combustion engine power systemcomprising: compressing air to provide a pressurized air stream;evaporating water into the pressurized air stream at multiple locationsalong a fluid flow path between a compressor and the combustion chamber,using waste heat from the power system, to produce a pressurizedsteam-air mixture; burning fuel with the pressurized steam-air mixturein a combustion chamber to produce useful power and waste heat, theburning also producing a steam-containing exhaust stream; expanding thesteam-containing exhaust stream through an expander to produce an outputpower in excess of the power required to provide the pressurized airstream; and taking-off excess power from the expander.
 54. A method ofoperating a combustion engine power system comprising: compressing airto provide a pressurized air stream; evaporating water into thepressurized air stream, using waste heat from the power system, toproduce a pressurized steam-air mixture; burning fuel with thepressurized steam-air mixture in a combustion chamber to produce usefulpower and waste heat, the burning also producing a steam-containingexhaust stream, the fuel comprising a fuel mixture comprising a firstfuel component and a second fuel component, the first fuel componentfacilitating burning of the second fuel component in the presence ofsteam; expanding the steam-containing exhaust stream through an expanderto produce an output power in excess of the power required to providethe pressurized air stream; and taking-off excess power from theexpander.
 55. The method of claim 54, wherein the first fuel componentcomprises hydrogen.
 56. The method of claim 55, further comprising: atleast partially reforming a fuel using at least one of heat and steamfrom the engine exhaust to produce the hydrogen.
 57. The method of claim54, further comprising: at least partially reforming a fuel to providethe fuel mixture, where the first fuel component comprises product of areforming reaction, and the second fuel component comprises at least oneof partially reformed fuel and un-reformed fuel.
 58. The method of claim57, further comprising: transferring heat from the engine exhaust to afuel reforming reaction.
 59. The method of claim 57, further comprising:diverting a portion of the steam-containing engine exhaust to providesteam for a fuel reforming reaction.
 60. A method of operating acombustion engine power system comprising: compressing air to provide apressurized air stream; evaporating water into the pressurized airstream, using waste heat from the power system, to produce a pressurizedsteam-air mixture; burning fuel with the pressurized steam-air mixturein a combustion chamber to produce useful power and waste heat, theburning also producing a steam-containing exhaust stream; expanding thesteam-containing exhaust stream through an expander to produce an outputpower in excess of the power required to provide the pressurized airstream; taking off excess power from the expander; and recoveringcondensed steam from the engine exhaust before the exhaust is dischargedfrom the system.
 61. The method of claim 60, further comprising:selectively applying a backpressure to the engine exhaust to facilitaterecovery of condensed steam from the exhaust.
 62. A method of operatinga combustion engine power system comprising: compressing air to providea pressurized air stream; evaporating water into the pressurized airstream, using waste heat from the power system, to produce a pressurizedsteam-air mixture; burning fuel with the pressurized steam-air mixturein a combustion chamber to produce useful power and waste heat, theburning also producing a steam-containing exhaust stream; expanding thesteam-containing exhaust stream through an expander to produceadditional power; and recovering condensed steam from the engine exhaustbefore the exhaust is discharged from the system.
 63. The method ofclaim 62, further comprising: selectively applying a backpressure to theengine exhaust to facilitate recovery of condensed steam from theexhaust.
 64. A method of operating a combustion engine power systemcomprising: compressing air to provide a pressurized air stream;evaporating water into the pressurized air stream, using waste heat fromthe power system, to produce a pressurized steam-air mixture; providingthe pressurized steam-air mixture and a fuel mixture to a combustionchamber, the fuel mixture comprising at least two fuel components,wherein a first fuel component facilitates burning of a second fuelcomponent in the presence of steam; burning the fuel mixture with thepressurized steam-air mixture in the combustion chamber to produceuseful power and waste heat, the burning also producing asteam-containing exhaust stream; and expanding the steam-containingexhaust stream through an expander to produce additional power.
 65. Themethod of claim 64, wherein the first fuel component comprises hydrogen.66. The method of claim 65, further comprising: at least partiallyreforming a fuel using at least one of heat and steam from the engineexhaust to produce the hydrogen.
 67. The method of claim 64, furthercomprising: at least partially reforming a fuel to provide the fuelmixture, where the first fuel component comprises product of a reformingreaction, and the second fuel component comprises at least one ofpartially reformed fuel and un-reformed fuel.
 68. The method of claim67, further comprising: transferring heat from the engine exhaust to afuel reforming reaction.
 69. The method of claim 67, further comprising:diverting a portion of the steam-containing engine exhaust to providesteam for a fuel reforming reaction.